I believe the real problem here is considering only the linear problem. If you instead look at this from the nonlinear point of view, I think it is much easier. I will go over it in detail when we get there.
Matt On Mon, Jul 7, 2008 at 4:27 PM, <[EMAIL PROTECTED]> wrote: >> On Mon, Jul 07, 2008 at 09:38:15AM +0100, Garth N. Wells wrote: >>> I've had a look at the code for applying Dirichlet boundary conditions >>> by eliminating constrained dofs, and I think that we need to do it >>> differently. A constrained dof should be indicated by a negative index >>> in the DofMap, and then eliminated during the main assembly loop. We >>> then need to do something for DiscreteFunction so that it can pick up >>> the boundary values, which should be pretty easy. >> >> PETSc handles this (skipping insertion for negative indices). >> >> http://www-unix.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/Mat/MatSetValues.html >> >> Does Epetra do the same? >> >> -- >> Anders > > > Sorry if I am stressing the wrong points here. I dont mind negative indices. > The way I understand it is, to simplify it to a 2x2 system: > > First, after element matrix and vector calculations are done we have > > [ a00 a01 ] [x0] = [b0] > [ a10 a11 ] [x1] = [b1] > > enforcing x0 = u0 gives > > [ 1 0 ] [x0] = [u0] > [ a10 a11 ] [x1] = [b1] > > then the symmetric variant of this system: > > [ 1 0 ] [x0] = [u0] > [ 0 a11 ] [x1] = [b1 - a10 u0 ] > > Assembling only negative indices in this example > would give the system: > > [ a11 ] [x1] = [b1 - a10 u0 ] > > Still a10 and u0 is needed for calculating the right hand side. > > Hence, negative indicies only remove the identity part, which > seems harmless enough. > > Using only negative indicies will however result in some work during post > processing if one like to see the solution also on the boundary. > > Kent > > _______________________________________________ > DOLFIN-dev mailing list > [email protected] > http://www.fenics.org/mailman/listinfo/dolfin-dev > -- What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead. -- Norbert Wiener _______________________________________________ DOLFIN-dev mailing list [email protected] http://www.fenics.org/mailman/listinfo/dolfin-dev
